The present invention relates to a process of fabricating a semiconductor device having contact holes with different depths.
A prior-art process of semiconductor device fabrication is shown in FIG. 2. First, an insulating film 2 (such as SiO.sub.2) for device isolation and a diffusion layer 3 are formed on a semiconductor substrate 1. Then, an insulating film 4 consisting of BPSG (boro-phosphosilicate glass) for example, is formed by CVD (chemical vapor deposition), and heat-treatment is then conducted to cause the insulating film 4 to flow thereby planarizing the surface. An opening 5 which will become a contact hole is then formed, and ion implantation is proformed through the opening 5. Another heat treatment is then conducted so that the slope of the wall of the contact hole will become gentle. An Al-Si alloy layer 6 which will become an interconnection (wiring conductor) is formed by sputtering, and interconnection pattern is then formed. This completes the fabrication of the semiconductor device.
A problem associated with the above process of fabrication is that as the degree of integration of the semiconductor device is increased, with the diameter of the opening 5 being reduced, and the aspect ratio (ratio depth/diameter of the contact hole) being increased, the step coverage of the Al-Si alloy layer 6 becomes poor, and the interconnection may be broken.
To eliminate this problem, techniques of filling the contact holes with metal have been developed. One of such techniques uses selective tungsten (W) CVD, and a semiconductor device obtained with this method is shown in FIG. 3. In this method, a device isolation insulating film 12 and a diffusion layer 3 are formed on a semiconductor substrate 11, in the same way as the method described with reference to FIG. 2, and then an insulating film 14 is formed and an opening 15 which will become a contact hole is formed. A tungsten film 16 is then formed by CVD such that there will be no step between the region of the contact hole 15 and the neighboring insulating film 14. An Al-Si alloy film 17 is then formed by sputtering, and then patterned by photolithography. In this method, the contact hole is filled with metal, so that interconnection breakage due to a poor step coverage can be avoided, and an interconnection with a high reliability can be obtained.
However, the contact holes may have different depths. For example, a contact hole for contact with a diffusion layer in the substrate is relatively deep, while a contact hole for contact with a gate electrode interconnection is relatively shallow. If the selective CVD is performed such thet there will be no step at the deep contact hole, the metal will flow over at the shallow hole. If, on the other hand, the selective CVD is conducted such that the there will be no step at the shallow contact hole, the thickness of the metal in the deep contact hole will be insufficient, and, when sputtering is performed on the resultant device, interconnection breakage may occur because of the poor step coverage.
The ion implantation into the contact is necessary to increase the impurity concentration at the surface of the contact Si in order to obtain a good ohmic contact. Moreover, to alleviate the sharpness of the steps, the interlayer insulating layer must have a good flowing property (tendency to flow easily when subjected to heat treatment), and is for example formed of BPSG. When heat treatment is conducted after the contact ion implantation, the walls surrounding the contact hole may also flow, and the opening is widened. When selective CVD is performed over the opening that has been widened, the resultant top surface will become convex. The flatness of the interconnection is degraded, and the degree of integration will be limited.